The present invention relates to a cylinder head gasket placed between two members such as a cylinder head and a cylinder block of an internal combustion engine.
When joint surfaces between a cylinder head and a cylinder block (cylinder body) of an automobile engine are sealed, a metal cylinder head gasket is placed between the cylinder head and the cylinder block to seal combustion gas, coolant water, and lubrication oil.
Such a cylinder head gasket has been changed from a laminated type having many layers of metal plates to a simple structure type composed of one or two metal plates for reducing a weight and production cost of an engine. Since only one or two structural plates are used, usable materials are limited for reducing a weight of an engine. A type and the number of sealing methods are limited, so that it is necessary to use a relatively simple sealing method.
As a weight and size of an engine have been reduced recently, the engine tends to have lower rigidity. Accordingly, it is difficult to obtain an even surface pressure around a cylinder bore upon sealing with the cylinder head gasket. That is, because of a structural problem around a cylinder bore, when the gasket is tightened, it is difficult to generate an enough sealing surface pressure at a low rigid portion, thereby causing gas leak at the portion.
When an engine has a smaller size in a longitudinal direction to reduce a size thereof, a space between the cylinder bores decreases, thereby increasing a temperature of the cylinder bores. As a result, the cylinder head and the cylinder block are deformed, thereby increasing a chance in which gas leaks between the cylinder bores. When a small amount of gas leaks and enters a water hole or oil hole, gas enters liquid such as water and oil, thereby causing a problem in circulating water and oil and cooling the engine. Since this causes a problem for the engine, it is necessary to provide a counter measure therefor.
Further, when a large sealing surface pressure is applied at a peripheral area just around a cylinder bore to ensure bore sealing performance, a deformation of a cylinder bore with low rigidity is promoted. Accordingly, a sealing member of a gasket does not function properly, and it is difficult to obtain good bore sealing performance.
A metal gasket with beads formed around cylinder bores (combustion chamber holes) has been proposed. The beads are connected or cross between the cylinder bores, and have straight portions between the cylinder bores. It has been proposed that an edge of the bead at a side of the cylinder bore is formed in a curved shape for smoothly connecting the straight portions between the cylinder bores and arc portions around the cylinder bores except the straight portions (see Patent Reference 1).
With this structure, a decrease in the surface pressure at the bead crossing section is prevented for small cylinder bores of a small engine. Also, a portion in which rigidity of the bead changes greatly around the bead crossing section is eliminated, thereby preventing gas leak at the bead crossing section.
However, in such a single bead structure with the straight portion, it may not be an enough solution for an increased temperature of the cylinder bore and lower rigidity of a recent engine.
Further, a portion between the cylinder bores is very small as compared with other portions, and a gasket may be formed of a single or two metal plates. Accordingly, only a limited sealing member can be disposed. As a result, it is difficult to combine complex sealing members. Even though it is possible, a number of production steps increases, thereby increasing production cost.
One of the inventors has proposed a metal gasket for an internal combustion engine. A bead with a wave shape in a plan view is provided around a liquid hole where it is difficult to obtain an enough sealing surface pressure, thereby securely sealing around the liquid hole (see Patent Reference 2).
Patent Reference 1: Japanese Patent Publication (Kokai) No. 2000-356266
Patent Reference 2: Japanese Patent No. 3026084
In view of the problems described above, an object of the present invention is to provide a cylinder head gasket for effectively preventing gas leak between cylinder bores of an engine with a small size and low rigidity.
Further objects and advantages of the invention will be apparent from the following description of the invention.